Motion Tracking Michael Donnerer, Sandeep Rakhra

Overview

  Introduction

  The ideal motion tracker

  Possible errors and problems

  Current Technologies

  Conclusion

Why do we need motion tracking?

  View control for immersive environments

  Cave™, head mounted displays

  Navigation in a virtual environment

  Intuitive object selection and manipulation

  Tracked gloves

  Instrument Tracking

  Computer-aided surgery

Why do we need motion tracking?

Orad TrackVision

 Motion Capture for Animation

 Movies & video games

  Augmented Reality

  Add virtual features to real world

  X-Ray vision, information

  Camera tracking – CGI effects

 Movies, news, sports (football – offside)

What would an ideal device look like?   Size

  Tiny would be nice

  Self contained

  No other parts on the user or on the environment

  6 Degrees of Freedom

  Position and orientation

What would an ideal device look like?   Accuracy in position and orientation

  Low latency & High update rate

  For example 1,000 Hz with latency less than 1ms

  Tenacity

  Track fast and far objects

What would an ideal device look like?   Immunity to occlusion

  Needing no clear line of sight

  Robustness

  Light, sound, heat, magnetic fields, radio waves, and other

trackers don’t interfere

What would an ideal device look like?   Autonomy

 Wireless, battery powered 3 years on coin sized battery

  Cost

  Cheap

Some Common Problems

Some Common Problems   Spatial distortion

  Constant difference between reported and actual position/

orientation

  Spatial jitter

  Changing difference between reported and actual position/

orientation

  Special case: Creep – constant increase in error over time

Some Common Problems   Slow update rate and/or high latency

  Latency Jitter

  Cycle to cycle variations in latency

Overcoming Latency   Latency can be a big problem

  Especially for fast objects

 Motion prediction - Azuma 1995[1]

  Extrapolate forward from most recent data

  “like driving a car by only looking in the rear-view mirror”

Mechanical Sensing Uses in object tracking

Mechanical Trackers   Conceptually simple approach

  Articulated arm

  Counterweights

  Physical linkage between tracked target and environment

  Use potentiometers, shaft encoders or optical technology

  Easy to get target position and orientation

SensAble PHANTOM® 1.5/6DOF

Mechanical Trackers  Most common uses

  Precision 3D digitization over a small area

  Precise measurements in engineering

SensAble PHANTOM® Desktop™ Faro Tech. FaroArms

Mechanical Trackers   Very precise and accurate

  High update rate and low latency

  Can be integrated with force-feedback

Mechanical Trackers  Generally limited to single target

  Small range of motion

  Typically under a cubic meter

Inertial Sensing Uses in object tracking

Inertial Trackers   First used in ships, submarines and airplanes in 50’s

  Early systems used gyroscopes far too heavy to be attached to a

person

 MEMS (microelectronic mechanical systems) inertial sensors in 90’s

  Small and lightweight

Xsens MTi

Inertial Trackers  Modern systems use “strapdown INS*”:

  Three orthogonal angular-rate gyroscopes

  Three linear accelerometers

  Resulting data double integrated to get position and orientation

 Gravity must be accounted for (g)

  (*inertial navigation system)

Nintendo Wii Remote

Inertial Trackers   Small size

  Completely self contained

  No line of sight

  Very low latency

  Typically a couple of milliseconds

  High update rate

  Thousands of samples per second

  Extremely low jitter

Animazoo IGS-190-M mobile

Inertial Trackers   Drift

  Bias error of just 1 milli-g in accelerometer causes drift of 4.5m

within 30s

  1 milliradian error coming from gyroscopes would produce same

effect

  Even very good gyroscopes that you wouldn’t wear on your

head drift by a milliradian within a short time

Inertial Trackers   Hybrids:

  Compass (Xsens MTi)

 GPS (Xsens MTi-G)

  Camera/Visual

 Others… Xsens Mti-G

Demo

Acoustic Sensing Uses in object tracking

Acoustic Trackers   Use transmission and sensing of sound waves

  Sonar

  Commercial system operate by timing the flight duration of a brief

ultrasonic pulsec

Intersense IS-900 Receiver and Transmitter components

(inertia & ultrasonic tracking)

Acoustic Trackers  Multi-pathing - direct and indirect signals

  Amplitude and phase of the signal can change wildly

  Pulsed time-of-flight acoustic overcomes this by waiting until signal

has arrived

Acoustic Trackers  Omni-directional system needs small speakers and microphones

  But efficiency proportional to the active surface area

  Line of sight required

  But somewhat tolerant to occlusions

Acoustic Trackers   Higher frequencies are beneficial to avoid interference

 Most noise sources fall off with increasing frequency

  Frequency dependent attenuation of sound in air starts to be

significant at 40kHz and dominant by 80 kHz

  Accuracy can be affected by

  Wind

  Uncertainty in the speed of sound

  Temperature, humidity

  About one-millimeter error per degree Fahrenheit at one

meter

Acoustic Trackers   Update rate limited by reverberation

  5 to 100 ms may be necessary because of echoes

  Update rates can be as slow as 10 Hz.

  Latency in general under 15ms

  Affected by distance

  Radio and Microwave tracking works in a similar way

Optical Sensing Uses in object tracking

  Rely upon measurements from reflected or emitted light

  Consist of two major components: light source and optical sensors

  Light sources can be passive or active

  Two common sensors used in industry…

Introduction

What is optical sensing in a nutshell? Tracking objects in space by measuring light intensity or processing images.

Analog Photosensors

Fast and Cheap

CCD (Charged Coupled Devices)

Computationally Expensive

Wii Remote & Sensor Bar Optical Sensing your mum’s familiar with…

  Capable of tracking hand motion in space

  Uses a infrared camera housed in the remote in conjunction with a

sensor bar emitting infrared via LEDs

  Inside-out methodology

  Houses accelometers, so inertial

tracking too

  Uses triangulation

  Outside-in methodology used in other

home brew Wii remote applications.

Reference Johnny Lee

  Not just a toy…

  Image based optical trackers use images from a camera and sample

for a predefined target or search for features

  CCDs fall into this category

  Microsoft researcher recently launched tracking software based on

this principle

Tracking Image Features

Demo

Advantages & Disadvantages   Outside-in or Inside-out? Both possible!

  Outside-in requires the tracked object to reflect or emit light. This

is cheaper and light-weight (good for tracking humans)

  Tracking volume increased by addition of more sources – fairly

cheap for optical sources

  Analog sensors are cheap and with active light sources provide fast

updates and good spatial precision

  Targets orientation is difficult to determine

  Require a clear line of sight! Multiple tracked objects interfere and

objects in the environment don’t help

Magnetic Sensing Uses in object tracking

  Function by measuring the strength of the magnetic fields using

magnetometers or current induced in an electromagnetic coil when a

changing magnetic field passes through the coil

  Three coils or magnetometers are embedded in a small unit that is

attached to whatever the system needs to track

  Measuring the magnetic fields generated on each of the three

perpendicular coils or magnetometers give rise to a 3D vector

  From vector it is possible to determine the position and orientation

of the sending unit

Introduction

What is magnetic sensing in a nutshell?

Tracking objects in space by measuring magnetic fields.

Advantages & Disadvantages   May experience interference operating in the vicinity of

ferromagnetic objects

  May experience interference operating in the vicinity of metal

object

  Need to work in a small volume to obtain accuracy

  Possible slow updates as coils subside before continuing

measurements take (not for magnetometers)

  No line of sight issues!

  One source can excite many sensors hence the ability to track many

objects form single source so cheaper in this sense. Reference Polhemus

  Component is small and light-weight

Hybrid Sensing Uses in object tracking

Hybrid Tracking

There is no single solution to tracking that suits all applications.

  We can select several solutions and combine their advantages to

produce better tracking results

  Example, the Wii Remote using optical and inertial sensing to fully

track hand movements

  The Wii Motion Plus provides more accurate tracking for rotational

movements

  Many more examples…

Conclusion

Tracking is essential for VE (Virtual Environments) but there is no silver bullet solution.

Questions?

References: [1] R. Azuma, Predictive Tracking for Augmented Reality, PhD dissertation, tech. report TR95-007, Univ. North Carolina,

Chapel Hill, Dept. Computer Science, 1995

General: Greg Welch, Eric Foxlin, "Motion Tracking: No Silver Bullet, but a Respectable Arsenal," IEEE Computer Graphics and

Applications, vol. 22, no. 6, pp. 24-38, November/December, 2002.